The present invention relates generally to a method for manufacturing a silicon single crystal using a pulling procedure based on a Czochralski (CZ) method, and more specifically, to a method for manufacturing a silicon single crystal by pulling a produced crystal while holding it at its upper end.
A silicon single crystal manufactured by the CZ method is widely used as the material for silicon wafers in semiconductor devices. FIG. 1 illustrates a conventional method for manufacturing a silicon single crystal by the CZ method. The seed crystal 8, held by the lower end of pulling-up shaft 6, is immersed in a silicon material melting solution 9 in a crucible 3, and pulled up by the shaft 6. The single crystal 10 is grown below the seed crystal 8. During this process, the crucible 3 and shaft 6 are rotated in the reverse direction from each other.
The seed crystal 8 is in the form of a slender rod, 10 mm in diameter. It is connected to a seed holder at the top thereof and immersed partly in the silicon material melting solution in the lower section. Immersing the seed crystal in a silicon material melting solution kept at a higher temperature causes a thermal shock to the seed crystal and leads to dislocation of the crystal. Therefore, after the seed crystal is immersed in the silicon material melting solution, it is drawn from the melting solution to reduce its diameter, and to prevent the crystal from being dislocated. This procedure is referred to as the xe2x80x9cneck process.xe2x80x9d The diameter of the neck is generally 5 mm or less, and is preferably 3 mm or less.
Conventionally, a single crystal of silicon manufactured with the CZ method is approximately 8 inches in diameter and weighs 100 kg. Recently, larger single crystals, 10-12 inches in diameter are increasingly in demand.
Single crystals rapidly gain weight, as their diameter increases. For example, a crystal having a 12 inch diameter may weigh 200 kg. Most of the weight of a single crystal is concentrated in the neck, which is the top section of the crystal and has the smallest diameter. The fracture strength of silicon is approximately 20 kg/mm2. Thus, in order to securely hold a silicon single crystal weighing 200 kg the drawn seed section needs to have a diameter which is larger than 5 mm. It is therefore difficult to pull up a single crystal having a 12 inch diameter.
One technique used to solve this problem pulls up the crystal without holding the neck. This technique is disclosed by Japanese Patent Publication No. 5-65477, and is illustrated in FIG. 2. In this technique, section 12 which mechanically holds the single crystal is formed. Section 12 is larger in diameter than the neck 11 and forms the crystal body section 13. Single crystal 10 can be pulled up while being held at the section 12 by a separate mechanical device, e.g., chuck mechanism. While Section 12 has a larger diameter in the upper section than in the lower section, the diameter of the lower section is still larger than the diameter of the neck. The diameter of the upper section is shaped like a knob, and the diameter of the lower section is constricted. Thus, section 12 may be referred to as a knob, or a constricted section. The knob section 12 is formed with a process similar to the one used to form the body of the single crystal 10. This process includes controlling the pulling up speed and the temperature of the silicon material melting solution by varying the output of the heater set around the crucible.
A single crystal of 12 inches in diameter can be stably pulled up when it is held by the knob section, rather than at the neck. However, forming of the knob section creates several problems associated with the use of a combination of controlling the pulling up speed and the heater output.
When pulling up a single crystal with the CZ method, an increase in the pulling speed decreases the diameter of the crystal, and vice versa. It is therefore possible to form a knob section by controlling pulling speed alone. However, because the pulling speed greatly decreases during the initial stage of the formation, formation of the knob section solely by controlling the pulling speed takes a long time and is therefore inefficient. Thus, using a combination of controlling pulling speed and heater output is necessary.
Increasing temperature at which the melting is performed decreases the diameter of the crystal. However, after the heater output level changes, e.g. the temperature of the heater is increased, it may take a significant amount of time for the temperature of the melting solution to increase and reach a target level. Thus, controlling the output by changing the output of the heater makes it impractical to form a knob section having a complex shape because such diameter should be controlled in a short time.
In other words, the shape of the knob section is largely determined by the chuck structure which holds it. There are some cases where the knob section cannot be mechanically held, depending on the type and extent of deviation of its shape from the target shape. Combining control of the heater output with control of the pulling speed, the reproducibility of the knob section shape is reduced, because of the large amount of time required to form a knob of the target shape. The knob section may also be difficult to mechanically hold, thereby yielding further inefficiencies.
Accordingly, a need exists for an efficient method for manufacturing a silicon single crystal, including forming a knob section in the upper section of the single crystal, by which the single crystal is mechanically held while being pulled up.
The method of the present invention for manufacturing a silicon single crystal by the CZ method forms a knob section in the upper end of the produced crystal. The manufactured single crystal is mechanically held by the knob section while it is pulled-up from the material melting solution in the crucible. The knob section is formed by controlling the rotational speed of the crucible.